Add open graph support

Hi,

I add a open graph code according to [pelican-bootstrap3].
I also add some additional meta tags like `og:site_name`.
To enable this setting, just set `USE_OPEN_GRAPH` to `True` in the config file.

For more open graph information, you can check: http://ogp.me
For debugging: https://developers.facebook.com/tools/debug/

[pelican-bootstrap3]: https://github.com/DandyDev/pelican-bootstrap3/blob/master/templates/base.html